Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths. When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman amplification scheme, although semiconductor optical amplifiers are also used in some situations. At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link is shared by all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine the channels into, and separate the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels lose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
Minimally, the optical signal generators, e.g., the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength control. Especially in systems with center-to-center wavelength channel spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any wander impairs the demodulation of the wandering signal at the far end receiver since the wavelength is now at a wavelength different than expected by the corresponding optical signal detector, and the wandering signal can impair the demodulation of spectrally adjacent channels when their spectrums overlap each other.
In addition to wavelength stability, optical signal generators that are tunable are also desirable for a number of reasons. First, from the standpoint of manufacturing, a single system can function as the generator for any of the multiple channel wavelength slots, rather than requiring different, channel slot-specific systems to be designed, manufactured, and inventoried for each of the hundreds of wavelength slots in a given WDM system. From the standpoint of the operator, it would be desirable to have the ability to receive some wavelength assignment, then have a generator produce the optical signal carrier signal into that channel assignment on-the-fly. Finally, in higher functionality systems such as wavelength add/drop devices, wavelength tunability is critical to facilitate dynamic wavelength routing, for example.
The present invention concerns a semiconductor laser wavelength locker system and specifically, the implementation of a semiconductor laser wavelength locker system that is compatible with a compact, integrated device.
In general, according to one aspect, the invention features a semiconductor wavelength locker system. The system has a differential wavelength filter system that applies multiple spectral filtering characteristics to a beam from a semiconductor laser system. A multielement detector is further aligned to the differential wavelength filter to detect a magnitude of the beam after being filtered by the multiple spectral filtering characteristics of the differential wavelength filter. The controller modulates a wavelength of the semiconductor laser in response to the differences in the magnitude of the filtered beam detected by the elements of the detector.
According to the preferred embodiment, the differential wavelength filter system comprises a stepped etalon. This etalon preferably has multiple steps, e.g., as two to three or more steps in the preferred embodiment, although more or less steps can be implemented in different configurations.
In one implementation, a beam splitter is provided outside of a laser cavity of the semiconductor laser system. This beam splitter provides a portion of the output of the laser as the beam received by the differential wavelength filter system.
In the preferred embodiment, in the control electronics, a mapper is preferably provided that correlates the differences of the filtered beam detected by the multi-element detector to a wavelength of the semiconductor laser. In this way, the system can be typically calibrated at manufacturing time, and the mapper programmed. Then, later, the laser system can determine its wavelength of operation on an absolute basis.
In one embodiment, a free spectral range of the differential wavelength filter is preferably between 50 and 150 nm. This provides relevance to modem WDM systems in which the C and L bands cover a range of about 1500 to 1600 nm.
However, in some implementations, in WDM systems requiring fine control, the free spectral range of the differential filter preferably has a smaller range of 10 nm or less than 1 nm in some embodiment.
Specifically, in some implementations, a coarse multi-element detector is further provided having a free spectral range on the order of about 100 nm, while a fine multi-element detector is provided with a free spectral range on the order of about 1 nm or less.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.